Herein, the term “adsorptive” is used to refer to a gas used which is adsorbed, the terms “adsorption isotherm” and “desorption isotherm” refer to data measured and plotted as curves of adsorbed amount versus pressure at a constant temperature onto a powder using physically adsorbed adsorptives, and the term “chemisorption isotherm” refers to similar curves of data measured and plotted but using chemically and selectively adsorbed adsorptives. This invention is related to the practice of measuring adsorption isotherms and desorption isotherms of solid powders, or the practice of measuring chemisorption isotherms of metal particles supported on solid powders, or the practice of measuring gas uptake curves of porous solid powders exposed to a step change in pressure. The measurement of an adsorption-desorption isotherm is also known as the BET (Brunauer, Emmett, Teller) measurement method. The measurement of a chemical adsorption isotherm is also known as the selective gas chemisorption method. The measurement of a gas uptake curve of porous solid powders at constant pressure is also referred to as a diffusivity measurement.
An adsorption isotherm can be analyzed to give the surface area of a powder sample. An adsorption-desorption isotherm can be analyzed to give the pore volume, pore shapes and pore size distribution of powder samples. A chemisorption isotherm can be analyzed to give the surface area and average particle size of catalyst particles on a support. A constant pressure gas uptake curve can be analyzed to give the diffusivity of a gas in a solid. Hereafter, adsorption refers to physical adsorption or chemical adsorption and an adsorption isotherm refers to an adsorption isotherm or a chemisorption isotherm.
The measurement of the gas amounts adsorbed on a solid sample as the pressure surrounding a sample increases gives the adsorption isotherm. Herein, the amount of gas adsorbed on a solid sample includes the gas adsorbed and condensed on the sample. The adsorption measurement is made by dosing amounts of gas into a sample cell, with the amount of gas in each dose known by measurement. The pressure in the sample cell will increase and stabilize at an equilibrium pressure if the rate of gas input is less than the rate of attainment of the equilibrium state. When adsorption occurs onto the sample, the equilibrium pressure is less than it would be without adsorption and the difference in pressure is used to calculate the amount of gas adsorbed on the sample. The amounts of gas adsorbed and the corresponding pressures in the sample cell are the data points of an adsorption or chemisorption isotherm. Successive data points are determined by dosing more gas into the sample cell causing the pressure therein to successively increase. The use of gas laws together with the pressure changes, volume and temperature allows the calculation of the amount of gas added to the free space of the sample cell, and the subtraction of this from the amount of gas dosed gives the adsorbed amount. Herein, free space refers to the space in the sample cell not occupied by sample and includes the volume of connecting conduits and valves. The dosing of gas into the sample cell can be either by a continuous flow method or an intermittent dosing method. A high resolution adsorption isotherm is one where the data points are as closely spaced as possible.
A desorption isotherm is measured similarly to an adsorption isotherm but with the difference that the measurement sequence is carried out in reverse, that is, beginning from where the sample is saturated with gas and then sequentially desorbing adsorbed gas. Herein, the amount of gas desorbed from a solid sample includes the gas desorbed or evaporated from the sample. The measurement commences with the state at the completion of the measurement of an adsorption isotherm, that is, the sample is saturated and the pressure is the saturated vapor pressure at the temperature of the sample (relative pressure of 1.0). The desorption isotherm is measured by measuring the amounts desorbed from the solid sample as the pressure around the sample is successively decreased. The amounts desorbed are measured by flowing gas from the sample cell to a gas chamber kept at a lower pressure, where knowing from measurements the amount of gas that has entered the gas chamber and the amount of gas that has been removed from the free space in the sample cell, the amount of gas desorbed from the sample is calculated from their difference. In this measurement, the rate of gas transfer must be less than the rate of attainment of pressure equilibrium in the sample cell. The amounts that remained adsorbed on the sample, and the corresponding equilibrium pressures in the sample cell are the data points of a desorption isotherm. The gas flow from the sample cell can be either by a continuous flow method or an intermittent dosing method. A high resolution desorption isotherm is one where the data points are as closely spaced as possible.
The basis for the above measurements is to know the total amount of gas supplied to or removed from the sample cell and the amount of gas in the free space of the sample cell. A method used to measure these amounts of gas is based on the use of a chamber of known volume and temperature and a sample cell of known free space and temperature, and the measurements of their pressures at different times to calculate by subtraction the changes in the pressures in the chamber and the sample cell. The gas laws are used to calculate the required amounts of gas. Their difference is the amount of gas adsorbed on the sample.
An embodiment of this method is due to Orr et al. in U.S. Pat. No. 3,850,040. This is an intermittent gas dosing method which used a shut-off valve between a chamber and a sample cell. To measure each adsorption isotherm point, the chamber is filled with gas to a pressure higher than in the sample cell, this pressure measured and then the shut-off valve connecting the sample cell and the chamber is opened until an equilibrium pressure is reached and this pressure is measured. The pressures in the chamber and sample cell before and after the valve is opened are used to calculate their changes in pressure. The amount of gas dosed into the sample cell and the amount of gas accumulated in the free space in the sample cell are then determined using gas laws. Their difference gives the amount of gas adsorbed. Then, the shut-off valve is shut and a repetition of this procedure is used to get the next data point. This is repeated point by point. To measure each desorption isotherm point, the procedure is similar except that the supply chamber is first evacuated to vacuum instead of being first filled with gas and the gas dose is from the sample cell to the chamber. In more modem variants of this method, separate pressure sensors are used to separately measure pressures in the sample cell and chamber. This embodiment has the disadvantage that the chamber has to be refilled or evacuated for each data point measurement.
Another embodiment of this method is shown in U.S. Pat. No. 5,637,810 to Connor. In this, in an adsorption measurement, a dosing manifold is first filled with gas. For each isotherm point, a dosing valve is used to admit a gas dose. The quantity of gas dosed is determined by the pressure in the dosing manifold and the volume of the dosing volume. This is repeated point by point. The dosing volume is made small and a ballast volume is used to provide flexibility in dose sizes. This embodiment has the disadvantage that the dosing manifold has to be refilled a number of times in the course of measuring the isotherms and the dosing volume has to be refilled for each data point measurement.
Additional disadvantages in these apparatuses are that the resolution and accuracy of the isotherm measurements are low. This is because the change in pressure is calculated from two measurements of the pressure. Thus, the dosing volume or chamber has to be of a small size to give measurable changes in its pressure. Due to this small size, the absence of an effective pressure control device in the device and intermittent dosing in discrete units, there are large changes in the pressure in the sample cell with each dose which result in a low resolution of the isotherm, that is, the points of the isotherm are spaced far apart. Also, the supply chamber must be refilled or evacuated many times in measuring an isotherm. This increases the experimental error since as each refill or evacuation gives rise to an experimental error, the errors are proportional to the number of refills. Also, experience is necessary to choose suitable pressures in the dosing volume or chamber and the operation or automation of the measurements requires the manipulation of many valves.
Other embodiments for measuring adsorption-desorption isotherms are shown in U.S. Pat. No. 4,762,010 to Borghard et al. and U.S. Pat. No. 5,109,716 to Ito et al. Borghard et al. used a flow restrictor while Ito et al. used a mass flow controller to control a continuous flow of gas between a supply chamber and a sample cell. The pressures are monitored and calculated changes in the pressures and calculations similar to those described above give the amounts of gas adsorbed or desorbed. As above, the changes in pressure are not themselves directly measured but rather calculated from the two measurements of the pressures at the start and end of some specified time periods. There is a need to ensure pressure quasi-equilibrium in the sample cell, and the flow rate must be kept very slow. One disadvantage in using this method is that the change in pressure in the supply chamber is very small relative to its magnitude, and due to the limited precision of pressure measuring devices, this limits the number of measured points in the isotherm, that is, there is limited resolution. Another disadvantage is again due to the change in pressure in the supply chamber being small relative to its magnitude. Due to the resulting limited precision in calculating the pressure change, this limits the volumetric size of the supply chamber to a small size which has to be small enough to give detectable changes in its pressure with the flow out of it of very small quantities of gas. A small size for the supply chamber has the disadvantage that, as gas is dosed from it, its pressure quickly falls and the supply chamber has to be refilled with gas to a higher pressure many times in the course of measuring an isotherm. The accuracy is limited since the error is proportional to the number of refills because of errors made at each refill. Automation is more complicated because there is also the need to automate the refilling of the supply chamber.
The speed with which a gas can reach the insides of porous solids is necessary information in many uses of powders, and the diffusivity of gases in porous solids is an important characterization of this property. An embodiment for measuring this type of diffusivity is shown in U.S. Pat. No. 4,762,010 to Borghard et al. The method measures the rate of gas uptake upon the application of a constant pressure. This method uses a procedure similar to the measurement of an adsorption isotherm but with the difference that the flow control device between the supply chamber and the sample cell is used to control the gas supply rate to keep the pressure in the sample cell constant. This again requires the control of the gas flow rate at a very slow rate (only the initial surge is fairly large). The disadvantages in the method of Borghard et al. are the same as discussed above, namely, a requirement to keep the flow rate very slow means that the change in pressure in the supply chamber is very small, and due to the limited precision of pressure measuring devices, this limits the number of measured points of the uptake curve, that is, there is limited resolution. Another disadvantage is that the limited precision of the pressure measuring devices limits the volumetric size of the supply chamber to a small size and there is then only a limited pressure range over which uptake curves can be measured.
In a further method to determine an adsorption isotherm, a reference cell is constructed to be virtually the same as a sample cell but used with non-adsorbing blanks in place of a sample, and this is used in conjunction with the sample cell. A supply chamber of known volume and temperature, and a sample cell and needle valve to control adsorptive flow between them constitute a sample subsystem. A second supply chamber with the matching reference sample cell and matching needle valve to control adsorptive flow between them constitute a reference subsystem. The flow rates of adsorptive in the sample subsystem and the reference subsystem are controlled to give matching pressure changes in the sample cell and reference cell, and the pressure difference between the supply chamber and the second supply chamber is measured to determine the amount of adsorptive adsorbed by the sample. A particular embodiment of this method is shown in a paper by Webb (Powder Handling and Processing, Volume 4(4), 1992, 439). The disadvantage of this method is that its construction and operation is quite difficult and expensive because the reference subsystem should precisely match the sample subsystem.
The object of the present invention is to provide an apparatus that does not have the disadvantages discerned above.